Since the Kyoto Protocol was adopted in December, 1997 in order to limit emissions of carbon dioxide which is a major reason for global warming, studies on a renewable and clean alternative energy source, such as solar energy, wind energy, and hydroelectric energy, have been actively conducted.
Among them, solar cells are attracting attention as a source of alternative energy. A solar cell is a device which converts solar energy into electric energy using a semiconductor device. Solar cells may be divided into an inorganic solar cell, a dye-sensitized solar cell, and an organic solar cell according to a material of a photoactive layer constituting the solar cell.
Currently, bulk type crystalline Si solar cells, which are a kind of inorganic solar cells, are used in most cases.
Generally, a silicon solar cell has a p-n junction structure formed in such a way that a p-type semiconductor layer is in contact with an n-type semiconductor layer. When this kind of solar cell is irradiated with light, excited electron-hole pairs, i.e., “excitons” are formed by the incident light. The excitons diffuse in an arbitrary direction to be separated into electrons and holes by an electric field generated in the p-n junction. At this time, the separated electrons move to the n-type semiconductor layer, holes move to the p-type semiconductor layer, and thus currents flow.
The electrons and holes recombine after a period of time after separation. Here, time taken for electrons or holes to recombine after they are generated is referred to as a carrier lifetime, and a distance over which electrons or holes travel until they recombine is referred to as a diffusion length.
As an example, a carrier lifetime of silicon is about 1 μs, and a diffusion length is about 100 μm to 300 μm.
However, the silicon solar cell has problems such as low photoelectric efficiency caused by optical loss in a surface of the solar cell, loss due to resistance of an electrode that collect electrons or holes, loss due to recombination of charges, etc. Accordingly, development of solar cells having high efficiency is required.
In order to increase efficiency of a solar cell, it is desirable to maximize a light incident area and a p-n junction area in which excitons are separated in a limited volume of a device. In addition, it is desirable to transfer electrons and holes to an electrode with less loss by reducing a diffusion distance in order to prevent electron-hole recombination.
Meanwhile, a nanowire has a high ratio of volume to area due to its one-dimensional structural characteristics. Accordingly, in a solar cell including the nanowire as a photoactive layer, diffusely reflected sunlight is likely to be re-incident on the device, and the direction in which electrons and holes can travel is limited to one direction. Thus, an average diffusion length of electrons and holes is reduced, which causes reduction in probability of loss of electrons and holes, resulting in increase of photoelectric efficiency of the solar cell.
FIG. 1 is a perspective view showing an existing nanowire-based solar cell.
Referring to FIG. 1, the existing nanowire-based solar cell includes a substrate 10, a plurality of silicon nanowires 20 perpendicularly arranged at constant intervals on the substrate 10, an insulating layer 30 filling between the nanowires, and electrodes 40 and 50 for connection to the outside.
The silicon nanowire 20 has a radial structure in which a p-type semiconductor layer 22, an n-type semiconductor layer 24, and a transparent electrode layer 26 are sequentially stacked from the inside. However, in the radial structure, since most of perpendicularly incident sunlight passes through areas other than a p-n junction portion at which the p-type semiconductor layer 22 is in contact with the n-type semiconductor layer 24, probability of photoelectric conversion is low, and thus improvement of photoelectric efficiency is limited.